Michael Deacon watches a spiteful Prime Minister's Question Time followed by
David Cameron's sombre statement on Pat Finucane.

Funny how quickly the air can change. From noon to 12.30 the House was ablaze with jeering and baying, cheap points and cheap jokes. Then, from 12.30 to 2pm, respectful silence, to hear respectful answers to respectful questions.

On days like this it feels as if, in some obscure parliamentary cubbyhole, there’s a panel of switches that control the chamber’s mood, tone and volume. Its operator, whoever that mysterious figure may be, works with scrupulous efficiency. Anger on… anger off. Maturity off… maturity on.

We’re watching the same politicians face the same opponents in the same chamber; yet in an instant they have metamorphosed from ranting blowhards to dignified statesmen.

Look at the difference today between Prime Minister’s Questions and, straight afterwards, the statement by the Prime Minister on Pat Finucane, the Belfast lawyer murdered in 1989 by loyalist gunmen with the collusion of British security forces.

PMQs was a blizzard of insults. David Cameron derided Ed Balls (“Like bullies all over the world he can dish it out but he can’t take it!”). Ed Miliband derided Mr Cameron (“I’ve heard everything when the boy from the Bullingdon lectures people on bullying! Have you wrecked a restaurant recently?”). Mr Cameron derided him back (“He’s catching the disease from the Shadow Chancellor of not being able to keep his mouth shut for longer than five seconds!”).

As ever it was like two schoolboys trying to grass each other up to the teacher. “He started it!” “No, sir, he started it!” “Don’t listen to him, sir! He ruined the economy first! I saw him!”

Then, as soon as the Finucane statement was under way, it was as if PMQs had never happened, the two leaders expressing their differences decently. It was all “Let me thank the Prime Minister”, and “I’m grateful to the Rt Hon Gentleman”.

How much better suited Mr Cameron is to the sombre statement than the shouting-match. As we saw in 2010 with his statement on Bloody Sunday, in September with his statement on Hillsborough, and again today, on such occasions he comes across as genuine, decent, sensitive. His tone is measured, his sorrow sincere, his response to disagreement reasonable.

“It pains me to read this report,” he said. “I’m so proud of our country, our institutions, of the police and our security service and what they do to keep us safe – and it is agony to read what happened.”

He explained why he was rejecting a public inquiry: he didn’t believe it could uncover more than Sir Desmond de Silva’s lengthy report had, plus it would “put a stay on any potential prosecution”. Several Labour MPs disputed his view, but always respectfully, and always with praise for his manner. “Notwithstanding the disagreement over an inquiry, can I commend him for the honesty of his statement,” said Peter Hain, the former Northern Ireland Secretary.

Then, at 2pm, it was time for a debate on NHS funding, and, somewhere in the bowels of Parliament, the mood-switch was flicked back to its default position.